Replaced Sophos XG - Sophos Central only sees old firewall SN

Question

We had an XG fail and we we have replaced it. 
 The original XG was registered with Sophos Central. 
 The replacement recently had its license updated accordingly. 
 Sophos Central shows live data gathered from recent traffic. Though it shows it from the original SN. 
 I tried to add the replacement firewall but it does not show as available - even though the local firewall says it's registered (and clearly data is reporting).

ChrisKnight · Accepted Answer

Be aware that removing the old firewall will also render any backups for that firewall in Sophos Central irretrievable, so make sure you download any needed backups prior to removing it. 
 Learnt this the hard way when Sophos Support helpfully removed the old firewall before I'd downloaded the latest backup to restore onto the replacement firewall - had to manually reconfigure the XG unit from scratch.